Liquid crystal display device having protrusions on the electrodes

ABSTRACT

A liquid crystal display device is constituted by a pair of oppositely disposed substrates each having a plurality of opposing electrodes, and a ferroelectric liquid crystal disposed between the substrates so as to form a plurality of pixels each composed by a combination of a pair of the opposing electrodes and the ferroelectric liquid crystal disposed therebetween. Each pixel is provided with regions of different polarity inversion threshold voltages, and at least one of the pair of opposing electrodes is provided with a plurality of regions having unevennesses at different densities including a region with a higher density of unevennesses corresponding to a region of a lower polarity inversion threshold voltage and a region with a lower density of unevennesses corresponding to a region of a higher polarity inversion threshold voltage.

This application is a division of application Ser. No. 08/099,054, filedJul. 29, 1993 now U.S. Pat. No. 5,495,352.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a liquid crystal device for use in adisplay apparatus, such as a television receiver, a computer terminal, avideo camera view finder, etc., particularly a liquid crystal devicewith an improved gradational display characteristic.

As an example of conventional liquid crystal display device, there isknown a liquid crystal display device based on an active matrix drivescheme using a TN (twisted nematic) liquid crystal. In this type ofdevice, thin film transistors (TFT) are disposed each at a pixel and ina matrix as a whole. For driving, a drive pulse is applied to the gateof a TFT to make the source-drain conductive to accumulate an imagesignal applied through the source at a capacitor on the drain side, sothat the TN-liquid crystal molecules at a pixel change their orientationdepending on the accumulated image signal to change the transmittance oflight therethrough. As a result, a gray-scale (gradational) display canbe effected by modulating the voltage of the image signal.

A device of such an active matrix drive scheme using a TN-liquid crystalhowever requires a matrix arrangement of TFTs each having a complicatedstructure and requiring a large number of production steps, thusrequiring a high production cost. Further, it is difficult to form athin semiconductor film of poly-crystalline silicon or amorphous siliconconstituting TFTs with a uniform characteristic over a wide area.

On the other hand, as an inexpensively producible liquid crystal displaydevice, one of the passive matrix drive scheme using a TN-liquid crystalis known. In this type of liquid crystal display device (panel),however, the proportion of time (duty factor) in which a selected pixelreceives an effective electric field during one picture (frame) scanningdecreases at a rate of 1/N as the number (N) of scanning linesconstituting the panel increases. Accordingly, there are involveddifficulties such that crosstalk occurs and a high contrast image cannotbe obtained. Further, as the duty factor decreases, it becomes difficultto control the gradation of each pixel by voltage modulation. In thisway, it is difficult to regard the liquid crystal display device of thepassive matrix scheme using TN-liquid crystal as a display device with alarge number of lines at a high density, such as a liquid crystaltelevision panel.

On the other hand, as a device solving such a basic problem of aconventional TN-liquid crystal, there is also known a liquid crystaldevice using a ferroelectric liquid crystal showing bistability. Sinceliquid crystal molecules in the ferroelectric liquid crystal deviceideally tend to be stabilized in either one of the two stable states anddo not assume an intermediate position, the ferroelectric liquid crystaldevice has been considered to be unsuitable for gradational display. Forthis reason, gradational display using a ferroelectric liquid crystaldevice has relied on a digital technique such as a pixel divisionscheme.

In such a gradational display method using a digital technique asdescribed above, one frame is divided into a plurality of sub-frameseach composed of plural pixels and, during one frame of writing,respective pixels within a sub-frame are supplied with electric fieldsof different duty factors for driving. In this case, in order to obtaina large number of gradation levels, one sub-frame has to be composed ofan increased number of pixels, so that the duty factor of each pixel ismade considerably small as the display screen becomes large.Accordingly, the liquid crystal material is required to show ahigh-speed responsiveness in order to obtain a high contrast. Further,one sub-frame requires a large number of driving electrodes for thegradational display. Further, a complicated operation circuit isrequired. These technical requirements may be too many in order to adoptthe gradational display method for a display apparatus of a large numberof gradation levels, such as a high-definition television (HDTV) set.

Other gradational display schemes using a ferroelectric liquid crystal(FLC) have been proposed by Japanese Laid-Open Patent Applications(JP-A) 59-193427, 61-166590, 62-131225, 64-77023, etc.

One suitable method of gradational display using an optical modulationmaterial inclusive of a ferroelectric liquid crystal has been disclosedin U.S. Pat. No. 4,796,980 entitled "FERROELECTRIC LIQUID CRYSTALOPTICAL MODULATION DEVICE WITH REGIONS WITHIN PIXELS TO INITIATENUCLEATION AND INVERSION" and issued to Kaneko et al. In this method, aninverted region and a non-inverted region are formed within a pixel, andthe transmittance through the pixel is controlled based on the arealratio between the regions. In order to form a locally inverted region,an alignment film having different uniaxial alignment control forces isused, or locally different electric fields are applied within a pixel.

When a ferroelectric liquid crystal is used in this method, however, itis necessary to control the applied voltages at a very high accuracysince the ferroelectric liquid crystal has a steep thresholdcharacteristic. Further, there is a tendency that the position and thedirection of growth of a polarity inversion region (domain) locallyoccurring within a pixel depending on an applied voltage are at random,so that it is not easy to obtain a linear voltage-transmittancecharacteristic.

Further, because of the temperature-dependence of the inversionthreshold of a ferroelectric liquid crystal, a desired gradation cannotbe reproduced as required in some cases. Further, it has been also foundthat a display image is affected by a previously displayed image and animproved reproducibility is required also in this respect.

SUMMARY OF THE INVENTION

In view of the above-mentioned technical problems, an object of thepresent invention is to provide a liquid crystal display device with asimple structure yet which is capable of realizing a larger number ofgradation levels than before.

Another object of the present invention is to provide a liquid crystaldisplay device capable of easily realizing a linearvoltage-transmittance characteristic and a stable gradational display.

A further object of the present invention is to provide a liquid crystaldisplay device with excellent temperature characteristics and excellentimage display reproducibility.

According to the present invention, there is provided a liquid crystaldisplay device, comprising: a pair of oppositely disposed substrateseach having a plurality of opposing electrodes, and a ferroelectricliquid crystal disposed between the substrates so as to form a pluralityof pixels each composed by a combination of a pair of the opposingelectrodes and the ferroelectric liquid crystal disposed therebetween;

wherein each pixel is provided with regions of different polarityinversion threshold voltages, and at least one of said pair of opposingelectrodes is provided with a plurality of regions having unevennessesat different densities including a region with a higher density ofunevennesses corresponding to a region of a lower polarity inversionthreshold voltage and a region with a lower density of unevennessescorresponding to a region of a higher polarity inversion thresholdvoltage.

According to another aspect of the present invention, there is provideda liquid crystal display device, comprising a pair of opposingelectrodes and a ferroelectric liquid crystal disposed between theopposing electrodes; wherein at least one of the opposing electrodes isprovided with a means for causing a lowering in polarity inversionthreshold voltage of the ferroelectric liquid crystal in the form of aline, and at least one of the opposing electrodes is provided with ameans for causing a change in shape of a polarity-inverted region indirection of the line of lowered polarity inversion threshold voltage.

According to another aspect of the present invention, there is provideda liquid crystal device, comprising a plurality of pixels eachcomprising a pair of opposing electrodes, and a ferroelectric smecticliquid crystal disposed between the opposing electrodes so as to developbistable states,

the liquid crystal device being equipped with a halftone signalapplication means for applying a halftone signal between the opposingelectrodes,

each pixel being provided with locally different inversion thresholdvoltages so as to develop a polarity-inverted region corresponding tothe halftone signal in a stripe pattern substantially parallel tosmectic layers of the liquid crystal.

According to another aspect of the present invention, there is provideda liquid crystal device, comprising a plurality of pixels eachcomprising a pair of opposing electrodes, and a ferroelectric smecticliquid crystal disposed between the opposing electrodes so as to developbistable states,

wherein at least one of the opposing electrodes is provided withstripe-shaped unevennesses extending in a direction such that saiddirection forms acute angles with two molecular orientation directionsin the bistable states of the liquid crystal, the smallest one of theacute angles in terms of absolute value not exceeding a cone angle ofthe ferroelectric liquid crystal in chiral smectic phase.

According to another aspect of the present invention, there is provideda liquid crystal optical device: comprising a plurality of pixels eachcomprising a pair of opposing electrodes and a ferroelectric liquidcrystal layer disposed between the opposing electrodes, wherein at leastone opposing electrode is coated with a fine particle-dispersion layercontaining electroconductive ultrafine particles having an averageparticle size of 30-300 Å and further an alignment control layerdisposed to allow a local direct contact of the fine particle dispersionlayer with the ferroelectric liquid crystal layer.

According to further aspect of the present invention, there is provideda liquid crystal optical device: comprising a plurality of pixels eachcomprising a pair of opposing electrodes and a ferroelectric liquidcrystal layer disposed between the opposing electrodes, wherein at leastone opposing electrode is coated with a layer of minute unevennessesproviding an average stepwise difference of 30-300 Å, and a means forproviding unevennesses having a stepwise difference larger than that ofthe minute unevennesses and disposed at a pitch larger than an averagethickness of the ferroelectric liquid crystal layer.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a liquid crystal deviceaccording to the invention.

FIGS. 2(a)-(d) are schematic top views of a pixel in a liquid crystaldevice according to the invention.

FIG. 3 is a schematic top view of another example of a pixel in a liquidcrystal device according to the invention.

FIG. 4 is a graph showing a voltage-transmittance characteristicaccording to the invention.

FIGS. 5A and 5B are a schematic top view and a sectional view,respectively, of a liquid crystal device illustrating a state of domainformation in a liquid crystal device.

FIGS. 6(a)-(d) are schematic top views of a liquid crystal deviceshowing a state of domain growth.

FIG. 7 is a schematic illustration of a one-dimensionally grown domainand a two-dimensionally grown domain.

FIGS. 8A and 8B are graphs showing a free energy change.

FIGS. 9(a) and (b) are schematic views showing changes with time ofdomains.

FIG. 10 is a graph showing voltage-transmittance characteristics of aone-dimensional domain and a two-dimensional domain.

FIG. 11 is a schematic top view showing a display state of a liquidcrystal device according to an embodiment of the invention.

FIG. 12 is a schematic top view showing a display state of a liquidcrystal device.

FIGS. 13(a), (b) and (c) are schematic views for illustrating a liquidcrystal device according to an embodiment of the invention.

FIGS. 14(a)-(d) are schematic top views illustrating a display state ofa liquid crystal device.

FIGS. 15A and 15B (a)-(a) are schematic views for illustrating domaingrowth in a liquid crystal device.

FIG. 16 is a liquid crystal molecule potential diagram in a liquidcrystal device according to an embodiment of the invention.

FIG. 17 is a graph showing a correlation between inversion thresholdvoltage and projecting stripe pitch in a liquid crystal device used inthe invention.

FIGS. 18A and 18B are a schematic sectional view and a partialenlargement thereof of a liquid crystal according to the invention.

FIG. 19 is an equivalent circuit diagram of a liquid crystal device.

FIGS. 20(a) and (b) are schematic illustrations of rubbing directions ofa liquid crystal device.

FIGS. 21-23 are respectively a graph showing a voltage-transmittancecharacteristic of a liquid crystal device.

FIGS. 24(a) and (b) are schematic views for illustrating a liquidcrystal device according to an embodiment of the present invention.

FIG. 25 is a top illustration of an island pattern in Example 1 of theinvention.

FIG. 26 is a sectional view of an AA-AA' section in FIG. 25.

FIGS. 27(a) and (b) are illustrations of rubbing directions in a liquidcrystal device.

FIG. 28 is a driving voltage waveform diagram applied to a liquidcrystal device.

FIG. 29 is an illustration of various inverted domains depending ondifferent applied voltages in Example 1 of the invention.

FIG. 30 is a graph showing a voltage-transmittance characteristic inExample 1.

FIG. 31 is a sectional view of an island pattern adopted in Example 2 ofthe invention.

FIG. 32(a)-(b) are illustrations of a stripe pattern used in Example 3of the invention.

FIG. 33 is a sectional view of a B-B' section in FIG. 32(a).

FIG. 34 is an illustration of pixels showing domains formed byapplication of different voltages.

FIG. 35 is a schematic illustration of a pixel in Example 4 of theinvention.

FIG. 36 is an illustration of a pixel pattern used in Example 5.

FIG. 37 is a sectional view of an AB-AB' section in FIG. 36.

FIG. 38 is an illustration of polarity-inverted domains observed inExample 5.

FIG. 39(a)-(b) are illustrations of a pixel pattern used in Example 6.

FIG. 40 is a sectional view of a BB-BB' section in FIG. 39(b).

FIG. 41 is an illustration of polarity-inverted domains observed inExample 6.

FIG. 42 is a sectional view of a pixel in Example 7.

FIG. 43 is a plan view of pixels in Example 7.

FIG. 44(a)-(b) illustrations of polarity-inverted domains observed inExample 7.

FIG. 45 is a sectional view of a pixelin Example 8.

FIG. 46 is a schematic plan view of a stripe pattern used in a liquidcrystal display device of Example 9.

FIG. 47 is a sectional view of a C-C' section in FIG. 46.

FIG. 48 is a schematic illustration of rubbing directions.

FIG. 49(a)-(b) are schematic illustrations of inverted domains atdifferent voltages in Example 9.

FIG. 50 is a schematic plan view of a stripe pattern used in a liquidcrystal display device of Example 11.

FIG. 51(a)-(b) are schematic illustrations of inverted domains atdifferent voltages in Example 11.

FIG. 52 is a graph showing a voltage-transmittance characteristic.

FIG. 53 is a schematic plan view of a stripe pattern used in a liquidcrystal display device of Example 13.

FIG. 54(a)-(d) are schematic illustrations of an example of inverteddomains at different voltages in Example 13, and the accompanyingvoltage diagram.

FIG. 55(a)-(d) are schematic illustrations of another example ofinverted domains at different voltages in Example 13, and theaccompanying voltage diagram.

FIG. 56(a)-(d) is a set of schematic views for illustrating a cellstructure of a liquid crystal display device.

FIG. 57 is a graph showing another example of a voltage-transmittancecharacteristic curve.

FIG. 58 is a block diagram of a display apparatus including a liquidcrystal display device (panel).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to a suitable embodiment of the present invention, a minuteinverted region is formed based on a region within a unit pixel and agray-scale or intermediate gradation level is displayed by controllingthe areal size of the inverted region.

More specifically, according to a first embodiment of the presentinvention, there is provided a liquid crystal display device,comprising: a pair of oppositely disposed substrates each having aplurality of opposing electrodes, and a ferroelectric liquid crystaldisposed between the substrates so as to form a plurality of pixels eachcomposed by a combination of a pair of the opposing electrodes and theferroelectric liquid crystal disposed therebetween;

wherein each pixel is provided with regions of different polarityinversion threshold voltages, and at least one of said pair of opposingelectrodes is provided with a plurality of regions having unevennessesat different densities including a region with a higher density ofunevennesses corresponding to a region of a lower polarity inversionthreshold voltage and a region with a lower density of unevennessescorresponding to a region of a higher polarity inversion thresholdvoltage.

In the present invention, it is preferred that the above-mentionedunevennesses are provided to both substrates, and the shape of anunevenness (pattern) may preferably be a projecting island or aprojecting stripe (or a corresponding indentation) but need not berestricted thereto. The unevennesses may be provided directly to atransparent film electrode, or a conductor or insulator formed on thetransparent electrode. Each unevennesses may preferably have a height(or depth) of 5-1000 nm, more preferably 50-300 nm. Each pixel may havea region having unevennesses at a higher density and a region havingunevennesses at a lower density. The unevennesses (projections orindentations) may preferably be spaced from each other by 2-1000 μm,more preferably 5-100 μm.

The above-mentioned regions having unevennesses at a higher density andat a lower density correspond to the regions having a higher and a lowerpolarity inversion threshold voltage, respectively, in the presentinvention.

Hereinafter, the principle of gradational display is described withreference to FIGS. 1-4.

FIG. 1 is a schematic sectional view of a liquid crystal deviceincluding a pair of substrates 11 and 12 of, e.g., glass, respectivelycoated with transparent film electrodes 13 and 14 and alignment films13a and 14a of, e.g., polyimide, and a ferroelectric liquid crystaldisposed between the substrates including molecules with axesrepresented by arrows 15. In case where the transparent film electrodes13 and 14 are provided with unevennesses, the liquid crystal molecularaxes 15 are disordered at the edge of an unevenness. In other words, atsuch an edge (region 16 surrounded by a dotted circle), molecular axes15 are inclined at a large angle with respect to the surfaces of thesubstrate to cause an increased interaction between the spontaneouspolarization and the electric field, thus being liable to causemolecular inversion. This phenomenon is particularly noticeably observedin case where the edge shape is sharply formed.

FIGS. 2(a)-2(d) are schematic top views of a liquid crystal device andillustrate states of inverted domain regions within a cell including atransparent film electrode with island-like projections 21 of a constantsize at different pitches (or spacings). The pitch is graduallyincreased from FIG. 2(a) to FIG. 2(c), and FIG. 2(d) shows a case withno projection. In all the cases, the cells are assumed to be initiallyoccupied with a black region 22 entirely. When such four types of cellsare supplied with identical voltages, the respective cells are providedwith inverted domains 23, of which the areas are in the order of(a)>(b)>(c)>(d). Further, in the cells (a)-(c), the inverted domains 23are formed at positions corresponding to the projections but, in thecell (d), the inverted domains 23 are formed at random. From FIGS.2(a)-2(d), it is clear that surface modification with unevennessesprovides a lowered threshold voltage at the modified parts and it isalso understood that, in a region provided with a high density of suchmodifications, the threshold voltage is lowered not only at theprojections but also at intermediate parts between the projections. Thissuggests that the lowering in threshold voltage is caused not only by anincrease in effective voltage due to a smaller cell gap at theprojections but also by a change in dynamic characteristic of the liquidcrystal in the vicinity of the modified parts. Accordingly, such anunevenness modification can be obtained not only by projections as shownabove but also by indentations. In this way, by providing an unevennesspattern onto a transparent film electrode, it becomes possible tocontrol the position of inversion domains and, by controlling thedensity of unevennesses, it is possible to vary thevoltage-transmittance characteristic.

As a variation of the cells shown in FIG. 2, FIG. 3 shows a unit pixelin which island-like projections 21 are disposed at locally differentdensities along with a state of instantaneous inversion. Morespecifically, in the pixel shown in FIG. 3, projections are disposed ata higher density in an upper left region and at a lower density in alower right region.

FIG. 4 shows voltage-transmittance characteristics represented by fourdashed curves LA, LB, LC and LD corresponding to the four cells (a)-(d)of FIG. 2, respectively, showing threshold voltages Vth.a, Vth.b, Vth.cand Vth.d satisfying Vth.a<Vth.b<Vth.c<Vth.d and saturation voltagesVs.a, Vs.b, Vs.c and Vs.d satisfying Vs.a<Vs.b<Vs.c<Vs.d. Herein, adifference in threshold voltage (e.g., Vth.b-Vth.a) is larger than acorresponding difference in saturation voltage (E.g., Vs.b-Vs.a).Accordingly, if a gradation display characteristic is represented by afactor γ=saturation voltage/threshold voltage (Vs/Vth), i.e.,γa=Vs.a/Vth.a, γb=Vs.b/Vth.b, γc=Vs.c/Vth.c and γd=Vs.d/Vth.d,respectively, for the cells (a)-(d) in FIG. 2, a relationship ofγa>γb>γc>γd holds. Accordingly, if one pixel is provided withunevennesses disposed at locally different densities as shown in FIG. 3,it is possible to not only control the position of inverted domains butalso provide a desired γ-characteristic as represented by a solid linein FIG. 4. Further, as a relationship of(Vth.d-Vth.c)<(Vth.c-Vth.b)<(Vth.b-Vth.a) holds, i.e., a largervariation in threshold voltage is attained in a higher unevennessdensity region, a desired gradation characteristic can be accomplishedwithin a small area by disposing regions with different unevennessdensities within one pixel, thus decreasing the area of one pixel. Thevariation in threshold voltage particularly noticeably caused when awriting pulse of 100 μs or shorter, particularly 40 μs or shorter, isapplied so that it is possible to obtain a sufficient gradationcharacteristic even if the number of scanning lines is increased.

Another embodiment of the present invention will now be described. Inthis embodiment, the growth of a domain is controlled so as to improvethe linearity of voltage-transmittance characteristic (γ-characteristic)and also the reproducibility thereof. More specifically, the inverteddomain is designed to be enlarged preferentially in a certain direction.

FIGS. 5A and 5B are a plan view and a sectional view (of A-A' section inFIG. 5A) of a liquid crystal cell showing a state of domains extendingat random in a planar direction. Referring to the figures, the cellincludes a pair of glass plates 11 and 12 respectively provided withopposite electrodes 13 and 14 of transparent conductor films andalignment films 13a and 14a of, e.g., polyimide. An optically blackdomain 22 is formed of liquid crystal molecules having upwardspontaneous polarization denoted by white arrows in FIG. 5B, andoptically white domains 23 are formed of liquid crystal molecules havingdownward spontaneous polarization denoted by black arrows in FIG. 5B.

As shown in FIG. 5B, a white or black domain may be regarded as almostuniform in the direction perpendicular to the substrates 11 and 12, andthe ratio between the number of molecules having downward spontaneouspolarization to the number of molecules having upward spontaneouspolarization (optically, an overall transmittance through a pixel)correspond to the two-dimensional areal ratio between the white domainsand the black domains.

FIGS. 6(a)-6(d) show four views of a cell at four points of time(t=Δt-4Δt) when the cell is continually supplied with a voltage in onedirection. As shown in FIG. 6, white (or black) domains grow (or shrink)while the external voltage application is continued. Accordingly, bycontrolling the magnitude or duration of the applied voltage, it ispossible to control the white domain/black domain areal ratio within onepixel, thus effecting a gradational display.

Incidentally, the appearance of the domain growth (or shrinkage) greatlyvaries whether it is accompanied with a two-dimensional shape change orone-dimensional shape change.

FIG. 7 schematically illustrates a one-dimensional domain change and atwo-dimensional domain change.

If explanation is made with reference to a straight-forward case,one-dimensional domain change refers to a shape change causedprincipally or only by a growth/shrinkage of length L while the width Ddoes not remarkably increase as the length L increases. On the otherhand, two-dimensional domain change refers to a shape change representedby a change in R as shown in FIG. 7.

Now, a free energy G possessed by each domain under voltage applicationis considered. As the elastic energy of a domain at a boundary wall isproportional to the boundary wall area and the electric energy isproportional to the volume of the domain, the following equations withα, β, k1, α' and γ are given:

G=αL-βL=k1×L for one-dimensional domain,

G=α'R-γR² for two-dimensional domain.

FIGS. 8A and 8B are graphs illustrating the free energy changes. As isunderstood from FIG. 8A, according to a two-dimensional domain change, adomain having a radius smaller than a critical value R₀ disappears undercontinual voltage application. Further, with reference to FIG. 9illustrating one-dimensional domain change and two-dimensional domainchange both under low-voltage application (a) and under high-voltageapplication (b), the number of one-dimensional domains does notsubstantially change from the initial state to the steady state bothunder low-voltage application and high-voltage application. However,with respect to two-dimensional monodomains, small domains present inthe initial state of, particularly, low-voltage application disappearunder continuation of the voltage application. In this way, in case ofvoltage application near the threshold voltage to two-dimensionaldomains, the transmittance cannot be readily changed but provides avoltage-transmittance characteristic as represented by a dashed line inFIG. 10. Such a characteristic is disadvanageous for gradational displaycompared with one-dimensional domains.

It can be seen from the above that in order to effect a gradationaldisplay by using a ferroelectric liquid crystal, it is preferable tocontrol one-dimensionally changing domains rather than two-dimensionallychanging domains.

According to this embodiment of the present invention, one-dimensionallychanging domains are controlled to effect gradational display.

For the above purpose, the polarity inversion threshold voltage of aferroelectric liquid crystal may be decreased in the form of a line soas to facilitate the polarity inversion along the line, thus promotingone-dimensional domain change as described above.

The means for causing a lowering in polarity inversion threshold voltageof the ferroelectric liquid crystal in the form of a line may forexample comprise linearly arranged island-like unevennesses or stripeunevennesses arranged in parallel.

An unevenness thus formed functions to cause an alignment disorder offerroelectric liquid crystal molecules particularly in the vicinity of acorner thereof and instabilize the portion, thus resulting in adecreased threshold voltage. In the case of island-like unevennesses,due to the closeness between unevennesses, the above-mentioned alignmentdisordered portions appear to be in continuity, thus forming a line oflowered threshold voltage. On the other hand, in the case of stripeunevennesses, lines of lowered threshold voltage are formed along thestripe unevennesses.

Each island-like or stripe-shaped unevenness (particularly, projection)may preferably have a height of 50-10000 Å, further preferably 500-3000Å, and the adjacent unevennesses may preferably be spaced from eachother by 1-50 μm. The island-like unevenness may have a shape of arectangle, a circle or any other shape with a preferable size of, e.g.,1-50 μm in side length or diameter. The stripe unevenness may preferablyhave a width of φ1-50 μm.

On the other hand, the means for causing a change in shape of apolarity-inverted region in the direction of the line of loweredthreshold functions to cause a one-dimensional change in combinationwith the above means for causing a lowering in polarity inversionthreshold voltage in the form of a line, and may for example comprisethe following.

First of all, the means may comprise island-shaped or stripe-shapedunevennesses along a line or lines of the decreased threshold voltage.In this case, it is possible to apply a voltage preferentially in thedirection of the line of decreased threshold voltage, thereby developingstripe-like domain changes.

The island-shaped or stripe-shaped unevennesses may preferably have aheight of generally 50-10000 Å, more preferably 500-10000 Å.

As another measure, it is preferred to form a gradually decreasing orincreasing gradient of electric field applied to the ferroelectricliquid crystal along the line of decreased threshold voltage. If such ameasure is taken with respect to the opposing electrodes, it is possibleto gradually elongate a polarity inverted region along the line ofdecreased threshold voltage from a higher electric region to a lowerelectric region. An example of opposing electrode adopting such ameasure may comprise island-like or stripe-shaped unevennesses disposedwith different spacing, e.g., sparse to dense or dense to sparse, alongthe line of decreased threshold voltage. Another example of such anopposing electrode may be one having a slope along the line of decreasedthreshold voltage or one equipped with a bias voltage application meansalong the line of decreased threshold voltage.

An opposing electrode having such island-like or stripe-shapedunevennesses provides a higher electric field-applying region in itsregion with a higher density of unevennesses and a lower electric fieldapplying region in its region with a lower density of unevennesses.Further, an opposing electrode having a slope provides a higher electricfield-applying region at a part providing a higher projection (i.e., apart providing a smaller spacing with its counter electrode) and a lowerelectric field-applying region at a part providing a lower or recessedpart (i.e., a part providing a larger spacing with its counterelectrode). Further, an opposing electrode equipped with a biasvoltage-application means provides a gradually increasing or decreasingelectric field strength applied to the ferroelectric liquid crystaldepending on the direction of applying the bias voltage.

Still another embodiment of the present invention will be describedhereinbelow. In this embodiment, image display with a betterreproducibility may be realized by improving the alignmentcharacteristic of a liquid crystal material.

A halftone display (gradational display) is effected by using a liquidcrystal display device according to this embodiment comprising a pair ofopposing electrodes and a ferroelectric smectic liquid crystal disposedbetween the opposing electrodes so as to develop bistable states, and bydeveloping a polarity-inverted region corresponding to a halftone signalapplied between the opposing electrodes in a stripe patternsubstantially parallel to smectic layers of the ferroelectric liquidcrystal.

More specifically, the liquid crystal display device according to thisembodiment comprises a plurality of pixels each comprising a pair ofopposing electrodes, and a ferroelectric smectic liquid crystal disposedbetween the opposing electrodes so as to develop bistable states,

the liquid crystal device being equipped with a halftone signalapplication means for applying a halftone signal between the opposingelectrodes,

each pixel being provided with locally different inversion thresholdvoltages so as to develop a polarity-inverted region corresponding tothe halftone signal in a stripe pattern substantially parallel tosmectic layers of the liquid crystal.

More preferably, at least one of the electrodes is provided with astripe unevenness pattern forming acute angles with two molecularorientation directions in the bistable states of the liquid crystal sothat the smallest one in terms of an absolute value of the acute anglesis set to be at most the cone angle of the ferroelectric liquid crystalin the chiral smectic phase.

According to this embodiment, the size and density of domains are causedto respond well and with a good reproducibility to applied voltages, sothat it is possible to obtain a linear voltage-transmittancecharacteristic easily and stably, thus allowing an excellent gradationaldisplay.

It is possible to form a domain responding well to an applied voltage byrelying on substantially only a combination of reliable techniques ofsetting of a liquid crystal alignment state and formation of aunevenness pattern, thus allowing an analog gradational display withgood controllability.

FIG. 11 is a schematic view for illustrating a liquid crystal displayapparatus according to this embodiment.

More specifically, FIG. 11 schematically shows a pixel 1 wherein, withina region (black region) 22 comprising liquid crystal molecules in one oftheir bistable states as the initial state, a plurality of stripe-shapeddomains (white regions) 23 polarity-inverted to the other of thebistable states of liquid crystal molecules. On the other hand, FIG. 12shows a pixel 1 wherein polarity-inverted domains 23 are formed not in astripe form but in a disorderly manner. In this case of stripe-shapeddomains 23, identical domains appear at a very high probability inresponse to application of an identical signal. Further, the length anddensity of the stripe domains are linearly changed depending on thewaveheight and/or pulse durations of the signal. Accordingly, a goodcontrollability of transmittance is attained suitably for gradationaldisplay.

In this embodiment, the above-mentioned one-dimensionally changingdomains appear up to a certain number and thereafter the domains grow soas to increase their width. More specifically, in case of a low appliedvoltage, a small number of domains appear. In case of a high appliedvoltage, the number of domains increases and/or the widths of thedomains are enlarged. In order to cause such a domain growth, it isparticularly preferred to appropriately set the longitudinal directionof the stripes and the liquid crystal molecular orientation direction ina manner as described hereinafter.

On the other hand, in the case of FIG. 12, repetitive application ofidentical signals is liable to develop domains with different shapes andsizes on each application, thus resulting in a poor reproducibility.Accordingly, the resultant domains do not completely correspond to thewaveheight and/or pulse width of the applied signal.

The ferroelectric liquid crystal used in the present invention maypreferably comprise a liquid crystal assuming a chiral smectic C phase,H phase, I phase, etc., particularly preferably a chiral smectic Cphase.

As for a pair of opposing electrodes constituting a pixel in a liquidcrystal cell of the present invention, at least one of the opposingelectrodes may preferably comprise a transparent conductor, suitableexamples of which may include tin oxide, indium oxide and indium tinoxide (ITO).

The halftone or gradation signal used in the present invention may be asignal with a modulated waveheight (pulse height), a signal with amodulated pulse width (inclusive of a modulated pulse number), or asignal with a waveheight and a pulse width both modulated. The halftonesignal generating means for generating such halftone signals may beprepared by a semiconductor integrated circuit. It is preferred to use asystem wherein a pair of electrodes are supplied with independentsignals so as to provide a halftone signal as a combination of thesignals.

By appropriately selecting the liquid crystal material, electrodematerial and alignment state in the cell, a polarity-inverted region inresponse to an applied halftone signal is developed as stripe-shapedregions substantially parallel to the smectic layer direction of theferroelectric liquid crystal.

In this embodiment of the present invention, it is preferred to providestripe-shaped unevennesses (preferably, projections) onto the liquidcrystal side of at least one electrode. Such a stripe-shapedunevennesses may be formed in a uniform shape or ununiform shapes withinone pixel. Moreover, it is possible to divide one pixel into sub-pixelsso that each sub-pixel is provided with stripes of a uniform shapewithin the sub-pixel, and different sub-pixels are provided withdifferent shapes of stripes. Such a stripe unevenness may be formed byan electrode per se or an alignment film thereon, or by an additionalmember for providing such a stripe. In any case, the stripes may formedeasily by a combination of film formation and patterning.

It is preferred that the stripe direction is set to form acute angleswith two molecular orientation directions in the bistable states of theliquid crystal so that the smallest one in terms of an absolute value ofthe acute angles is at most the cone angle of the ferroelectric liquidcrystal. The molecular orientation directions (n1, n2) in the bistablestates may be determined from extinction positions when the cell in amemory state is observed through cross-nicol polarizers. The cone angleH may be determined from extinction positions when the cell underapplication of bipolar rectangular pulses of amplitude of 20 volts andfrequency of 10 Hz is observed through cross nicol polarizers. Thus, ahalf of the angle between the two extinction positions is taken as thecone angle H. The layer normal direction L of the liquid crystal may bedetermined by a bisector of the angle between the two extinctionpositions for the above-mentioned measurement of the angle H.

FIG. 13 is a schematic view of one pixel 1 provided with stripeprojections 21 for showing a state of inverted domains 23 as viewed froma direction perpendicular to the pixel 1. The inverted domains 23 areformed in stripes. Every inverted domain 23 in FIG. 23 substantiallycorresponds to a direction perpendicular to the layer normal directionL, i.e., extends in a direction of smectic layers. There are typicallytwo cases including one wherein stripe domains 23 extend across aplurality of stripe projections 21 as shown at (a), and another casewherein stripe domain 23 extend clearly in an identical direction onlyin recesses between stripe projections 21 like ladder steps. If stripeprojections are formed with an identical pitch and with a narrowprojection width, e.g., at most one half of the helical pitch of theliquid crystal, it is possible to preferentially result in state (a). Ineither case of (a) and (b), the inverted domains change their widthdepending on the applied signal while keeping the number thereof.Further, once the domain walls are formed after the signal application,there is observed little instability that the domains are enlarged orshrinked thereafter and there is observed little fluctuation intransmittance among pixels.

In the case of no stripe unevennesses, the inverted domains do not havea clear directionality or appear regularly regarding positions, thusresulting in fluctuation in transmittance among pixels as described withreference to FIG. 2. Further, even after the formation of domain walls,there is observed instability of domains.

Incidentally, in order to develop domains extending in the direction ofsmectic layers at a good controllability, it is preferred that the angleformed between the layer normal direction and the direction of stripeunevennesses is made at most two times the cone angle H of the liquidcrystal.

In order to obtain the above-mentioned layer structure of the liquidcrystal, it is possible to modify both electrodes with stripeunevennesses. In this instance, if the stripe directions on both sidesare made substantially parallel, both the alignment state and thegradational display state are improved.

In order to develop the stripe domains, it is preferred to apply arubbing treatment for controlling the alignment state. In this case, itis possible to apply such a rubbing treatment to both electrode sides oreither one electrode side with or without stripe unevennesses. In caseof rubbing a face with stripe unevennesses, it is preferred that therubbing direction is substantially parallel to the stripe direction. Aparticularly good gradational display state is accomplished when therubbing direction and the stripe unevenness direction forms an acuteangle not greater than H. In case of rubbing both substrates, bothsubstrates may be applied to each other so that their rubbing directionsare parallel and identical (parallel rubbing) or parallel and oppositeto each other (anti-parallel rubbing). A further better gradationalcharacteristic is attained if the stripe domains are formed in a pitchwhich is in a range of from about one half to two times the helicalpitch of the liquid crystal measured in a thick cell, i.e., in a bulkstate, and is generally about 1-20 μm, and the stripe domain length isat least two times the stripe domain width (generally at least 2 μm,preferably at least 20 μm) in a halftone display state of 50%transmittance.

As described above, the threshold voltage can be better controlled ifthe stripe unevennesses are disposed with different pitches in a pixel.FIG. 14 shows four pixels with stripe projections 21 at differentpitches on one electrode side, thus showing different states of inverteddomains, wherein L denotes the layer normal direction. In FIG. 14, at(a) is shown a case of a small pitch, at (b) is shown a case of a mediumpitch, at (c) is shown a case of large pitch, and at (d) is shown a caseof no stripe projections. The cells in all the cases are assumed to beinitially occupied with a black region 22 entirely. When such four typesof cells are supplied with identical voltages, the respective cells areprovided with inverted domains 23, of which the areas are in the orderof (a)>(b)>(c)>(d). Further, in the cells (a)-(c), the inverted domainsextend in the direction of layers but, in the cell (d), the inverteddomains 23 are formed at random. The above phenomena may be attributableto that not only the stripe unevennesses cause stripe inverted domainsbut also a strong dynamic interaction occurs between stripes in case ofa small pitch so that the resultant inverted domains are easily latched.

A change with time of pixel states at the time of signal application isexplained with reference to FIGS. 15A and 15B. In FIG. 15B, four pixelstates in a left column corresponds to a case of a small stripe spacingat the time (a)-(d) in FIG. 15A, and four pixel states in a right columncorrespond to a case of a large stripe spacing, respectively when acompletely new pulse is applied. In either case, the parts on the stripeprojections are first inverted at time (b). At time (c), a region ofdifferent transmittance has started to be seen in the case of a smallstripe spacing but, in the case of a large spacing, the transmittance islowered as a whole. Finally, at time (d), stripe domains are latched inthe case of small stripe spacing, but, in the other case, the stripedomains do not remain as inverted domains. In other words, it isconsidered that a difference in threshold occurs depending on adifference in dynamic interaction in a transient state. The domainforming process described above is noticeably observed when a voltagewith a pulse width of 100 μs or shorter is applied.

In addition to the above cause, it is also considered that, in additionto an alignment control force due to rubbing, an alignment control forceis exerted by a stripe projection, so that a certain asymmetry instability is caused between the bistable states of the ferroelectricliquid crystal. FIG. 16 shows a potential curve of bistable states. InFIG. 16, a stable state at PA corresponds to a case where the molecularaxes are oriented closer to the stripe direction and one at PBcorresponds to case where the molecular axes are oriented farther fromthe stripe direction. Herein, the abscissa represents a parameterrelating to a spontaneous polarization, e.g., a phase angle of Cdirector. In case of a small stripe spacing, this alignment controlforce is large to provide a large asymmetry as represented by a dashedline in FIG. 16. In other words, a larger threshold is given in case ofwriting from PA to PB and a smaller threshold is given in case ofwriting from PB to PA.

Accordingly, in actual gradational display, it is preferred that themolecular orientations are reset to one of the two stable orientationdirections in the bistable states, which is farther from the directionof the stripe unevennesses inclusive of setting of the polarizer andanalyzer. In this case, the inversion threshold value becomes larger ina region of a smaller stripe pitch than in a region of a large stripepitch. If the switching is in a reverse direction, of course, themagnitudes of threshold voltage are reversed.

FIG. 7 shows a change in inversion threshold when the stripe pitch isvaried. Solid lines represent cases of writing from the PA state to PBstate in FIG. 16 and dashed lines represent cases of writing from the PBto PA state.

Herein, the reset state is assumed to be a black state, followed byapplication of a polarity pulse for writing in white domain. As isunderstood from FIG. 17, the pulse width may preferably be at most 100μs, particularly preferably 40 μs or less, more specifically around 20μs as shown in FIG. 17. The fact that such a short pulse width issufficient, means that a sufficient gradational display characteristiccan be obtained even when the number of scanning lines is increased.

As described above, in a preferred embodiment of the present invention,a good gradation control characteristic is retained without any problemeven if the number of pixels is increased by disposing a pluralityregions with different pitches of stripe unevennesses.

The stripe pitch used in this embodiment should be selected at anoptimum value depending on the liquid crystal and the cell thicknessused so as to be at least one half of the helical pitch, morespecifically 3-50 μm. Further, the pitch should desirably be at leastthree times, preferably at least ten times, a smaller one of the widthsof the projection and indentations constituting the unevennesses. Thestepwise difference between the projection and the indentation mayselected within an extent of retaining a uniform alignment state, i.e,at most one half of the cell gap and generally in the range of 10 nm-500nm. The unevenness pattern need not be perfect rectangular.

Incidentally, it is of course possible to effect a thermal control inorder to retain the cell within a desired temperature range. Thealignment film for liquid crystal alignment may comprise an organic filmof, e.g., polyimide, polypyrrole, and polyvinyl alcohol, or an obliquelyevaporated film of, e.g., SiO.

In the above-described embodiments, disposition of unevennesses andorientations of liquid crystal molecules are optimized to improve thegradational display characteristic. In the following embodiment, aliquid crystal device is provided with improved electrical properties soas to remove factors adversely affecting the gradational display.

One difficulty encountered in gradational or halftone display arisesfrom a reverse electric filled effect induced by spontaneouspolarization of the ferroelectric liquid crystal per se. Thus, anelectric field formed by an internal ion localized corresponding to thespontaneous polarization can presumably unstabilize a desired halftonestate and cause a hysteresis of optical response in response to anexternally applied voltage. More specifically, a "black state" or a"white state" with a respective spontaneous polarization direction isfurther accompanied with a respective localization of ion stabilizingthe respective spontaneous polarization direction. Due to the differencein polarity of the localized ion, even if an identical voltage isapplied after a short period of resetting (erasure into "black"), theresultant halftone state can vary depending on whether the previousstate is "white" or "black" having received a different voltage.

According to still another embodiment of the present invention, there isprovided a ferroelectric liquid crystal optical device with goodalignment characteristic and capable of providing a uniform and stablehalftone with easy gradational display designing.

More specifically, according to this embodiment, there is provided aliquid crystal optical device: comprising a plurality of pixels eachcomprising a pair of opposing electrodes and a ferroelectric liquidcrystal layer disposed between the opposing electrodes, so as to form anoptically gradational state in response to an external voltage betweenthe opposing states depending on a given gradation signal, wherein atleast one opposing electrode is coated with a fine particle-dispersionlayer containing electroconductive ultrafine particles having an averageparticle size of 30-300 Å and further an alignment control layerdisposed to allow a local direct contact of the fine particle dispersionlayer with the ferroelectric liquid crystal layer.

FIG. 18A is a schematic sectional view of a liquid crystal opticaldevice according to the present invention and FIG. 18B is a partiallyenlarged view thereof.

Referring to FIGS. 18A and 18B, the device includes a pair oftransparent substrates 11 and 12 of, e.g., glass or quartz, providedwith transparent electrodes 13 and 14 of, e.g., ITO, SnO₂ or In₂ O₃, oneof which can be replaced with a metal electrode of, e.g., Al or Au. Inthis embodiment, electrode plates 10a and 10b composed of the abovemembers are coated with layers 3a and 3b containing electroconductiveultrafine particles having an average particle size of 30-300 Å, andthen further coated with a film of, e.g., polyimide, polyamide orpolyvinyl alcohol formed by spin coating or by the LB process, followedby aligning treatment to form alignment control layers 4a and 4b.

Between the alignment control layers 4a and 4b is disposed aferroelectric liquid crystal (FLC) layer 5 preferably having amoderately larger spontaneous polarization (e.g., 1 nC/cm² -100 nC/cm²).

In contrast with an ordinary insulating film of, e.g., polyimide orpolyvinyl alcohol, the fine particles-dispersion films 3a and 3b of thisembodiment can have a sufficiently lower resistivity to lower the timeconstant of the entire cell and can retain a large film capacitanceC_(A) compared with the above-mentioned insulating film if the thicknessthereof is on the same order. As a result, it is possible tosufficiently minimize the effect of conductivity hindrance due to thepressure of an insulating layer which has been considered to beproblematic.

More specifically, the above-mentioned problematic phenomena oftransmittance change and hysteresis are caused by a change in ΔQps(localized ionic charge) due to a difference in spontaneous polarizationin various halftone states of FLC as described above. In thisembodiment, by using a small resistance of the fine particle-dispersionfilm, it is possible to provide a small synthetic time constant [R_(A)R_(LC) (C_(A) +C_(LC))]/(R_(A) +R_(LC)) derived from an artificialequivalent circuit as shown in FIG. 19, which time constant may beapproximated to R_(A) (C_(A) +C_(LC)) in case of R_(A) <<R_(LC), therebyquickly relaxing the effect of fluctuation in ΔQ_(pS) as describedabove. Further, by retaining a large apparent film capacitance C_(A), itis possible to sufficiently minimize the value of ΔQ_(pS) /(C_(A)+C_(LC)) which is a factor representing the effect of ΔQ_(pS) on theliquid crystal partial voltage.

In order to provide a desirably short approximate time constant of about30 msec or shorter, i.e., equal to or shorter than one frame periodaccording to an ordinary motion picture rate, to the fineparticle-dispersion film while avoiding a voltage decrease or devicebreakage due to a short circuit caused by contamination with dust etc.,between the pair of electrodes, the fine particle-dispersion film maydesirably have a resistivity of 10⁴ ohm.cm-10⁹ ohm.cm, more desirably onthe order of 10⁷ ohm.cm. The fine particle dispersion can have anapparently increased capacitance by about one digit, i..e, an apparentpermittivity of 10-50, compared with a film having an identicalthickness of an ordinary insulating material. This is a desiredcharacteristic as described above.

Further, the alignment control layer disposed on the fineparticle-dispersion film is disposed in a thickness which is smallerthan the average particle size of the electroconductive fine particleswithin the fine particle dispersion film, so that the above-mentionedlow impedance effect is not hindered, i.e., desirably in a thickness of10-100 Å, preferably 10-50 Å, so as to provide a film rich in pinholesand allowing electrical contact between the liquid crystal and the fineparticle-dispersion film.

The ultrafine particles may for example comprise ITO or SnO₂ of whichthe electroconductivity may be controlled by doping with antimony (Sb),etc.

Further, if the alignment control layer is formed in a thickness of atmost 20 Å, an improved alignment characteristic is provided. In order toprovide a better alignment characteristic, it is preferred to use apolymer as a matrix or a binder resin.

A thin alignment control layer used in this embodiment exhibits a secondmajor function, i.e., a synergistic functional effect with the alignmentcharacteristic and gradation control characteristic given by theparticle size of the fine particle-dispersion film. This point will bedescribed hereinbelow.

An electrode substrate provided with a transparent electrode of ITO,etc., is generally accompanied with a naturally occurring unevennessfluctuation on the order of several tens to several hundreds Å. Whensuch an electrode plate is coated with a thin alignment control layerof, e.g., 30 Å, followed by aligning treatment and cell construction toprovide a parallel cell arrangement (FIG. 20(a)), a local alignmentirregularity or even an alignment defect can occur. Further, if such adevice is used in matrix gradational display drive, there can occur adifference in voltage transmittance characteristic among pixels,as shownin FIG. 22 or a poor gradational characteristic (γ characteristic).

In contrast thereto, if the above-mentioned fine particle-dispersionlayer is disposed according to the present invention, a relative longperiodical unevenness can remain but the respective minute portions arecovered with short periodical unevennesses to provide a good alignmentcharacteristic with little fluctuation among pixels, and also a goodgradation characteristic.

In the present invention, the fine particles used have an averageparticle size which is larger than an ordinary FLC molecule length (onthe order of 10-30 Å) but at most ten times the FLC molecules length.This is considered to be a reason why a good gradation characteristicand a smooth switching characteristic are attained without causingalignment defects.

The thickness of the alignment control layer used in this embodiment maybe at most about the above-mentioned short period, thus providing analignment uniformity and a good gradation characteristic.

In a specific example of the liquid crystal device according to thisembodiment, a cell was prepared by rubbing alignment control layers ("LQ1802") on a pair of substrates and stacking the substrates to each otherto form an anti-parallel cell (FIG. 20(b)), which was then filled withan FLC. In the cell immediately after the filling, the cell entirelycould show a splay alignment state in some cases. In such a case, whenthe cell was supplied with a rectangular AC voltage of 30 volts and 10Hz, a uniform alignment state of a high homogeneity could be obtainedwhile providing an increased apparent switching angle between memorystates of the FLC molecules.

When the cell was driven for gradational display, a good gradationcharacteristic was attained.

As a comparison, a cell was prepared by coating the same electrodesubstrates directly with 30 Å-thick polyimide films, followed by rubbingand stacking to each other, to form an anti-parallel cell. The cellcould only show clearly inferior alignment characteristic and gradationcharacteristic.

Incidentally, an anti-parallel cell obtained by stacking a pair ofsubstrates so that their rubbing directions were parallel and oppositeto each other on both sides of the liquid crystal layer could provided abetter γ-characteristic than that of a parallel cell having rubbingdirections which were parallel and identical (FIG. 23).

In order to provide a good film property, the fine particle-dispersionfilm may preferably be formed by the above-mentioned fine particlestogether with a binder or matrix resin which may be selected as showinga good dispersibility of the ultra fine particles or metal fineparticle. The film-forming binder or matrix resin may also be selectedfrom conventional alignment film materials, such as polyimide, polyamideand polyvinyl alcohol, or another material such as polysiloxane. Otherexamples may include electroconductive film-forming materials, such aspolypyrrole, polyacetylene, polythiophene, and charge transfer complexessuch as polyvinylcarbazole doped with tetracyanoquinodimethane (TCNQ).

In case where the binder or matrix resin is used, it is preferred thatthe fine particle-dispersion film contains about 10-90 wt. % of theultrafine powder.

The fine particle dispersion film may preferably be formed in arelatively small thickness of about 300-1500 Å.

It has been also found effective to apply a surface treatment forproviding a lower surface energy onto the alignment control layer as bya vapor deposition of a silane coupling agent within a non-excessiveextent for the purpose of stabilizing the gradation characteristic. Thesurface treatment is considered to be effective in enhancing the effectsof the above-mentioned short periodical unevenness provided by the fineparticle-dispersion film which has effects of smoothening the molecularmovement at the liquid crystal substrate boundary and removingdifficulties, such as local cell thickness change due to unnecessaryelastic distortion. This surface treatment is particularly effectivewhen the alignment treatment directions on both sides of the liquidcrystal layer are made anti-parallel, and also when an AC applicationtreatment is applied immediately after the liquid crystal injection orfilling so as to obtain a uniform alignment. This means that the surfaceenergy lowering treatment has a function of persistently stabilizing thelayer structure obtained by the AC treatment.

In the present invention, it is also possible to enhance the uniformityof gradation characteristic by intentionally combining relatively longperiodical unevennesses which have a height difference larger than thatof the short periodical minute unevennesses given by the fine particles,and are arranged at a pitch larger than an ordinary FLC layer thickness.

FIG. 24(a) is a partial sectional view of a pixel wherein longperiodical unevennesses 7 are intentionally provided by patterning ordeposition through a mask. FIG. 24(b) is a corresponding plan viewshowing an arrangement of 5 μ-square projections disposed at a pitch ofboth vertically and horizontally within a 200 μm-square pixel. Eachprojection may preferably have a height which is sufficiently small sothat the fine particle-dispersion layer 3b can be applied without asubstantial application irregularity thereon but is larger than theaverage particle size of the fine particles, and optimally on the orderof 300-5000 Å. The projections 7 may preferably be disposed at a pitchwhich is equal to or larger than the FLC layer thickness, preferably atleast several times the thickness, more specifically at least 0.5 μm,preferably about 1-50 μm, so as to show a synergistic effect with theshort periodical unevennesses. Such unevennesses may be formed with aninorganic insulating substance such as SiO₂ ; a metal such as Al, Ti orAu; a transparent conductive oxide, such as SnO₂, Tn₂ O₃, or ITO, or aresin such as polyimide or polyamide by a known patterning technique ordeposition technique. The long periodic unevennesses may be formed at aconstant pitch or different pitches within a pixel. One pixel may beprovided with a region where the unevenness are formed in a differentpitch. Further, the unevennesses may be formed at locally differentheights within a pixel. The unevennesses can also be formed in the formof lines or stripes, or islands.

In this case, the thin alignment control layer on the fineparticle-dispersion film can also be formed as a known obliqueevaporation layer of an inorganic substance, such as SiO or TiO₂desirably with a columnar length of at most 400 Å, instead of theabove-mentioned organic alignment layer. In this case, the unevennesseffect given by the lower fine particle-dispersion film is alsoexhibited and an electroconductive path is formed between the depositedcolumns, thus showing combined effects of long periodical unevennessesand short periodical unevennesses and providing a good gradationcharacteristic. In a specific example, a pair of substrates were coatedwith obliquely evaporated SiO columns and were stacked to each other sothat their SiO-evaporated directions were anti-parallel, followed byfilling with FLC. As a result, the thus obtained cell provided a gooduniform alignment state without applying an AC voltage as describedabove and realized a gradation characteristic with a high uniformity.

Further, the above unevennesses can also be formed by dispersion ofparticles so as to increase the uniformity among pixels.

For example, such unevennesses of particles may be formed by extending adispersion liquid containing core-shell structure particles having ahydrogel shell layer as disclosed in Polymer Preprints, Japan, Vol. 40,No. 11 (1991) 4090-4092, "Preparation of core-shell particles having ahydrogel layer, and surface characteristics thereof", and utilizing theproperty of self-aligning to precipitate of the particles, whereby anappropriate periodic arrangement structure can be formed. Details of theconditions for obtaining such core-shell structure particles aredescribed in the above reference. Briefly speaking, for example,core-shell structure particles suitable for production of periodicalarrangement of unevennesses may be formed by using latex particles asseed or core particles and by using N-isopropylacrylamide as atemperature-sensitive hydrogel shell-forming material. For example, thethus prepared dispersion liquid may be extended to dispose the depositedparticles of about 0.4 μm-diameter with an average particles spacing ofabout 1 μm to several μm on a transparent electrode substrate afterdrying. These particles may be affixed onto the substrate, e.g., by athermal treatment. Thereon, the above-mentioned electroconductive fineparticle-dispersion layer may be formed by coating.

In the present invention, it is also possible to first form theabove-mentioned electroconductive fine particle-dispersion layer, thenforming the alignment control layer, followed by aligning treatment, andthen dispersing the core-shell structure particles on the alignmentcontrol layer.

Examples

Hereinbelow, the present invention will be described based on examples,which however should not understood to restrict the present invention.Particularly, it should be understood that the disclosed members orelements can be replaced or modified within an extent of accomplishingthe objects of the present invention.

Example 1

FIG. 25 shows a pattern of pixels 1 used in this Example. Each pixel 1was provided with modifications 21 (island-like projections) formed on atransparent film electrode. The projections were disposed densely at acentral part and sparsely at a peripheral part. Each pixel had a size of200 μm-square, and each projection 21 had a size of 2 μm-square. Theprojections were disposed at a pitch of 2 μm at the central part and ata pitch of 15 μm at a peripheral part, respectively of the pixel.

FIG. 26 shows an AA-AA' section in FIG. 25. Glass plates 11 and 12 werecoated with 1500 Å-thick ITO films 13 and 14 which were formed into apixel pattern and provided with the above-described pattern ofprojections through two cycles of photolithographic steps. Theprojections had a stepwise height of about 700 Å. Thereon, 200 Å-thickpolyimide films were formed and rubbed to form alignment films 13a and14a. The-thus treated substrates were applied to each other so as toform a parallel cell as shown in FIG. 27(b) with a cell gap of 1.2 μmtherebetween, and the gap was filled with a ferroelectric liquid crystal(a blend based on a commercially available ferroelectric liquid crystal"CS-1014" available from Chisso K. K.). The cell showed a goodalignment. Between a pair of electrodes of the cell thus prepared, pulsevoltages as shown in FIG. 28 were applied to examine the opticalresponse characteristic. In this Example, Δt=50 μs, Vap=16-30 volts. Amatrix of 4×4 pixels with the initial state of black were examined byapplying different pulse voltages Vap for the respective columns toexamine the inverted domain shapes, the results of which are shown inFIG. 29. The transmittance fluctuation among pixels for each appliedvoltage was very small, and the occurrence centers (centers of gravity)of the inverted domains were all found at the central part of eachpixel. Further, as shown in FIG. 30, as a result of the surfacemodification with projections, the controllability of gradationaldisplay was improved.

Example 2

A liquid crystal cell was prepared by forming square projections similarto those in Example 1 with SiO₂. FIG. 31 shows a section of an electrodesubstrate used herein. A glass substrate 1104 was coated with a 700Å-thick ITO film 1103 and then with an 800 Å-thick SiO₂ sputtered film,which was then patterned photolithographically into square projections1101. Then, an alignment layer 1102 was formed thereon by obliqueevaporation of SiO₂.

Pairs of substrates thus treated were assembled to form both a parallelcell and an anti-parallel cell (FIGS. 27(a) and 27(b), wherein thearrows represent the oblique evaporation directions). Then, the cellswere examined with respect to the optical response characteristics. As aresult, the cells showed a good and uniform gradational displaycharacteristic similarly as in Example 1, whereas the anti-parallel cellshowed a more uniform gradational characteristic and a larger γ value.

Example 3

In this Example, stripe projections 1203 and 1204 were formed in pixels1201 and 1202 respectively. Each pixel had a size of 100 μm-square, andeach stripe projection had a width of 2 μm. The stripe projections werearranged at pitches gradually increasing from 2 μm (densest part) to 12μm (sparsest part). The upper substrate and the lower substrate weredisposed so that their stripe projections were perpendicular to eachother with respect to each pixel. FIG. 33 shows a B-B' section in FIG.32. Thus, a glass substrate 1301 was provided with 700 Åthick patternedITO electrodes 1302, thereon stripe projections 1303 formed bypatterning a 200 Å-thick Pt film, and then further coated with a 200Å-thick polyimide film 1304, which was then rubbed in the direction ofthe stripes on the upper substrate.

The thus treated pair of substrates were affixed to each other so as toform an anti-parallel cell. The cell thus prepared with 4×4 pixels ininitially black state was supplied with pulse voltages in the samemanner as in Example 1 with different amplitudes of pulse voltages Vapfor each column to examine the inverted domain shapes, the results ofwhich are shown in FIG. 34. As shown in FIG. 34, the inverted domainswere grown with an upper left part as the occurrence center (center ofgravity). The transmittance fluctuation among the pixels in each columnwas very small, and a display device with a uniform and rich gradationcharacteristic could be obtained.

Example 4

In this Example, stripe projections similar to those formed in Example 1were disposed at a high density at a central part and at a low densityat a peripheral part. FIG. 35 shows a schematic view showing a pixelpattern used in this Example. A lower substrate and an upper substratewere provided with stripe projections 1501 and 1502, respectively. In apixel of this Example, an inverted domain was grown from the center ofthe pixel as a domain growth center by application of a halftone displayvoltage. As the applied voltage increased, the domain area was enlargedwhile the center of gravity of the growing domain was always at thecenter of the pixel, thus effecting a good gradational display wasperformed on the entire device.

Example 5

FIG. 36 shows a pixel pattern used in this Example.

Each pixel 1 of 200 μm-square was provided with 2 μm-wide stripe-shapedunevennesses 21 disposed with a spacing of 10 μm.

FIG. 37 shows an AB-AB' section in FIG. 36.

Glass substrates 11 and 12 were provided with 1500 Å-thick films 13 and14 (as opposing electrodes) which were formed into a pixel pattern andprovided with the above-described pattern of unevennesses through twocycles of photolithographic steps. The unevennesses 21 had a stepwisedifference of about 200 Å. Thereon, 200 Å-thick polyimide films wereformed thereon and rubbed the direction of the stripe unevennesses toform alignment films 13a and 14a.

The thus-treated substrates were applied to each other with a cell gapof 1.2 μm therebetween, and the gap was filled with a ferroelectricliquid crystal (a blend based on "CS-1014"). In this way, two types ofcells, i.e., a parallel cell and an anti-parallel cell (FIGS. 27(b) and27(a)), were prepared by changing the direction of applying twosubstrates. Both cells prepared in this way showed a good alignment.

Between a pair of opposing electrodes 13 and 14 of each cell, pulsevoltages as shown in FIG. 28 were applied to examine the opticalresponse characteristic. In this Example, Δt=50 μs, Vap=16-30 volts. Amatrix of 2×2 pixels with the initial state of black were examined byapplying different pulse voltages Vap of about 20 volts to examine theinverted domain shapes, the results of which are shown in FIG. 38 (inthe case of an anti-parallel cell).

For the purpose of comparison, a cell with no stripe unevennesses wasalso prepared. However, the cell provided with stripe unevennessesshowed a clearly lower polarity inversion threshold voltage. The domainshape was generally linear and, under application of different voltages,the number and the length of the linear domains were principallychanged, so that one-dimensional domain change was confirmed.

The transmittance fluctuation among pixels was very small, and theoccurrence centers (centers of gravity) of the inverted domains were allfound at the central part of each pixel. Substantially the same resultswere obtained with respect to the parallel cell.

Further, a cell was prepared by providing the stripe unevennesses toonly one of the substrates 11 and 12. However, the cell showed a smallerdecrease in polarity inversion threshold voltage and a weakone-dimensional behavior.

Example 6

In this Example, an upper substrate was provided with stripeunevennesses similarly as in example 5, and a lower substrate wasprovided with stripe unevennesses of SiO₂ disposed at differentdensities within a pixel.

FIG. 39 shows a pixel pattern used in this Example.

Pixels 1201 on an upper substrate and pixels 1202 on a lower substratewere provided with stripe projections 1203 and 1204, respectively.

Each pixel had a size of 100 μm-square, and each stripe projection had awidth of 2 μm. The stripe projections on the upper substrate and thelower substrate were designed to intersect perpendicularly with eachother. On the lower substrate, the stripe projections were disposed witha spacing of from 2 μm (at the densest part) to 15 μm (at the sparsestpart),

FIG. 40 shows a BB-BB' section in FIG. 39.

The lower substrate 102 included a glass substrate 1301, opposingelectrodes 1302 of 700 Å-thick ITO film, 2000 Å-thick stripe projections1303 of patterned SiO₂ film and thereon a 200 Å-thick alignment film1304 of polyimide rubbing-treated in the direction of the stripeprojections 1303 (1204) on the lower substrate.

The upper substrate 1201 was prepared similarly as the lower substrateexcept that polyimide alignment film thereon was rubbing-treated in adirection perpendicular to the stripe projections.

The thus-prepared upper and lower substrates were applied to each otherso as to form a parallel cell with a cell gap of 1.6 μm and filled witha ferroelectric liquid crystal material showing a short helical pitch.The cell was supplied with an AC electric field of ±15 volts and 10 Hzwhile being cooled from smectic A phase to smectic C phase, whereby theresultant cell showed a good alignment.

The thus-prepared cell with a matrix of 4×4 pixels with the initialstate of black was examined by applying a different pulse voltage Vapfor each column to observe the inverted domain shapes, the results ofwhich are shown in FIG. 41.

The domains in each pixel showed a good one-dimensional behavior. At alow voltage, the respective pixels showed inverted domains only at theupper parts thereof so that it was confirmed possible to control theposition of domain occurrence by the pattern of SiO₂ stripe unevennessesprovided to the lower substrate, i.e., to provide a regulated thresholdvoltage distribution. The transmittance fluctuation among pixels at eachvoltage was very small, and a display device with a uniform and richgradation characteristic was obtained.

Example 7

FIG. 42 is a sectional view showing a pixel used in this Example.

An upper substrate 1601 was coated with a 500 Å-transparent opposingelectrode 1602 of ITO on which were further disposed island-shaped(square) projections 1603 of ITO by lifting-off and then a 100 Å-thickalignment layer 1604 of polyimide, followed by rubbing.

On the other hand, a lower substrate 1605 was provided with a slope orgradient within a pixel and coated with a transparent opposing electrode1606 of 500 Å-thick ITO, and an alignment film 1607 of 100 Å-thickpolyimide, followed by rubbing in the maximum gradient direction.

The thus prepared upper substrate 1601 and lower substrate 1605 wereapplied to each other with a spacer therebetween to form ananti-parallel cell, which was then filled with a ferroelectric liquidcrystal to obtain a liquid crystal cell.

FIG. 43 shows a plan view of the upper substrate 1601 including a pixel1701 and 10 μm-square projection 1702 of 1000 Å in height.

The cell was supplied with different values of voltage to examine theinverted domain shapes, the results of which are shown in FIG. 44. InFIG. 44, at (a) is shown a domain state at a low voltage application, at(b) is shown a medium voltage application, and at (c) is shown a highvoltage application. The inverted domains were generally linear, andshowed a one-dimensional behavior and a good gradation characteristic.

For the purpose of comparison, a cell was prepared in the same manner asabove except that the square projections 1702 were not provided to theupper substrate. The cell showed only a small lowering in polarityinversion threshold voltage, thus failing to provide a large Vsat/Vthratio between the threshold voltage Vth at a transmittance of 0% and thesaturation voltage Vsat at a transmittance of 100%, so that accurategradation control was impossible.

Example 8

FIG. 45 is a sectional view used in this Example.

An upper substrate 1901 was coated with a transparent opposing electrode1902 of 500 Å-thick ITO on which were further disposed island-shaped(square) projections 1903 of ITO by lifting-off and then an alignmentfilm 1904 of 100 μm-thick polyimide, followed by rubbing.

On the other hand, a lower substrate 1905 was coated with an opposingelectrode 1906 of 300 Å-thick high-resistivity ITO film showing a sheetresistivity of 1-100 MΩ. Further, on both sides of the opposingelectrode 1906 constituting a pixel, 1000 Å-thick Al electrodes 1907were disposed, and the entire electrodes were covered with an alignmentfilm of 100 Å-thick polyimide, followed by rubbing.

The thus-prepared upper substrate 1901 and lower substrate 1905 wereapplied to each other with a spacer therebetween to form ananti-parallel cell, which was then filled with a ferroelectric liquidcrystal to form a liquid crystal cell.

Then, one of the Al electrodes was grounded, and a bias voltage Vb ofabout 10 volts was applied to the other to develop a one-dimensionalpotential gradient in parallel with the rubbing direction in the pixel.

Then, the cell was supplied with different values of voltages betweenthe opposing electrodes 1902 and 1906 to examine the resultant inverteddomain shapes. As a result, the inverted domains caused at differentvoltages were substantially as shown in FIG. 44, and a good gradationcharacteristic was obtained.

Example 9

FIG. 46 shows a pixel pattern of a liquid crystal display deviceaccording to this Example. Each pixel 1 in size of 200 μ-square wasprovided with stripe projections 21 which were disposed in a width of 3μm and at a spacing of 5 μm on a transparent film electrode on asubstrate.

FIG. 47 shows a cell section along a C-C' line in FIG. 46. The cellincluded glass substrates 11 and 12 coated with 150 nm-thick ITO films13 and 14, the former being formed into a pixel pattern and the latterbeing formed into a pixel pattern and provided with the stripeprojections 21 through two cycles of lithographic steps. The stripeprojections were formed to have a height of about 70 nm. The ITO films13 and 14 were further coated with about 20 nm-thick polyimide films 13aand 14a, followed by rubbing.

Referring to FIG. 48, the rubbing was effected in a direction of r1 onthe lower substrate 12 provided with stripe projections and in adirection of r2 on the upper substrate 11 with no stripe projections. Asviewed from the upper substrate toward the lower substrate, thedirection r1 was set to form an angle of 0 degrees and the direction r2was set to form a clockwise angle of 10 degrees, respectively withrespect to the stripe direction.

Then, the substrates were applied to each other with a gap of 0.12 μm toform a parallel cell, which was then filled with a ferroelectric liquidcrystal (a blend based on "CS-1014") showing a cone angle in chiralsmectic C phase of about 15 degrees and chiral pitch of about 10 μm at30° C.

The thus-prepared liquid crystal cell showed a good alignment. In thecell, the direction of a bisector between the bistable directions ofliquid crystal molecules was in a direction forming a counterclockwiseangle of -2 degrees with respect to the stripe direction.

Between a pair of electrodes of the cell thus prepared, pulse voltagesas shown in FIG. 28 were applied to examine the optical responsecharacteristic. In this Example, Δt=50 μs, Vap=16-30 volts. A matrix of4×4 pixels with the initial state of black were examined by applyingdifferent pulse voltages Vap for the respective columns to examine theinverted domain shapes. As a result, the pixels supplied with differentvoltages showed pixel states as shown in FIG. 49. The inverted domainsextended in a direction of 88 degrees from the stripe direction. Thestripe domains were formed at a pitch of about 8 μm and in a lengthsubstantially crossing the pixel. The fluctuation in transmittance amongthe pixels at an identical voltage was very small, and the inversioncenter of gravity was constantly in the vicinity of each pixel.

Further, four additional cells were prepared in basically the samemanner as above except that the rubbing directions on the uppersubstrate with no projections were in a direction r2 of 10 degrees, 5degrees, 0 degrees and -5 degrees with respect to the stripe direction.Each cell showed good characteristics.

Further, an additional cell was prepared by providing similar stripeprojections to both substrates while selecting the rubbing directions ofr1 and r2 at 0 degrees and -8 degrees, respectively, and applying thesubstrates each other so that their stripe projections were parallel toeach other. The cell thus obtained showed substantially identicalresults.

Example 10

A liquid crystal cell was prepared in the same manner as in Example 9except that the rubbing directions r1 and r2 were set to angles of 0degrees and 180 degrees, respectively. The direction of the bisectorbetween the bistable states in the cell substantially coincided with thestripe direction. As a result of measurement of optical responsecharacteristics in the same manner as in Example 9, it was possible toobtain a uniform display with good gradation control characteristic.

Example 11

FIG. 50 shows a pattern of pixels and stripe projections formed on asubstrate used in a liquid crystal display device of this Example. Eachpixel 1201 of 200 μm-square was provided with 3 μm-wide stripeprojections 1202, which were disposed with a spacing of from 1 μm (atthe densest part) to 15 μm (at the sparsest part). A liquid crystal cellwas prepared by using the substrate otherwise (preparation method,liquid crystal material and cell gap) in the same manner as in Example aexcept that r1 and r2 were taken in substantially parallel directions.The cell showed a good alignment state.

The cell was provided with cross nicol polarizers set to a position sothat one of the bistable states remoter to the stripe direction provideda reset position (black state). The cell was then supplied withdifferent pulse voltages Vap to examine the inverted domain shapes, theresults of which are shown in FIG. 51. The inverted domains initiallyoccurred at the position with stripe projections disposed with thenarrowest spacing. The voltage-transmittance characteristic curve inthis Example is shown in FIG. 52. As shown in FIG. 52, it was possibleto take a large gradation control range (voltage width). Thetransmittance fluctuation among pixels at a constant a voltage was verysmall, and a display with a uniform and rich gradation characteristicwas realized.

Example 12

A liquid crystal cell was prepared basically in the same manner as inExample 11 except that the rubbing on the substrate provided with thestripe projections was omitted and the rubbing treatment on the othersubstrate alone was performed in a direction r2 at a counterclockwiseangle of -10 degrees. The cell provided a substantially identicalalignment state as in Example 11 and good optical responsecharacteristics similarly as in Example 11.

Example 13

FIG. 53 shows a pixel area 1501 of stripe projections 1502. Morespecifically, each pixel area 1501 of 200 μm-square was divided into 9regions each provided with stripe projections 1502 of 3 μm in width atrespectively different pitches. More specifically, 9 regions haddifferent stripe pitches so that the stripe pitch gradually increasedfrom 1 μm (region 1) to 9 μm (region 9). The stripe projections per sewere prepared in the same manner as in Example 9.

A lower substrate having a pixel and projection pattern as describedabove was combined with an upper substrate identical to the one used inExample 9 so as to prepare a liquid crystal cell otherwise in the samemanner as in Example 9. The resultant cell showed a good alignmentstate.

The cell was provided with cross nicol polarizers set to a position sothat one of the bistable states remoter to the stripe direction provideda reset position (black state). The cell was then supplied withdifferent pulse voltages Vap to examine the inverted domain shapes, theresults of which were schematically as shown in FIG. 54. The inverteddomains initially occurred in the region 1 with stripe projectionsdisposed with the narrowest spacing.

At the lower part of FIG. 54, there are also shown voltage-transmittancecurves obtained at regions A, B and C (1, 3 and 7 in FIG. 53),respectively.

Further, the position of the cross nicol polarizers was shifted so thatone of the bistable states of liquid crystal molecules closer to thestripe direction provided a reset position (black state). The cell wasthen supplied with different voltages Vap to examine the inverted domainshapes, the results of which were schematically as shown in FIG. 55. Theinverted domains initially occurred in region 9 with the stripeprojections disposed with the broadest spacing. At the lower part ofFIG. 55, there are also shown voltage-transmittance curves obtained atregions A, B and C (1, 3 and 7 in FIG. 53), respectively.

In any case, it was possible to form plural regions with differentinversion threshold voltages within one pixel, and it was possible torealize a display with a uniform and rich gradation characteristicsimilarly as in Example 11.

Example 14

A liquid crystal cell was prepared in the same manner as in Example 13inclusive of the use of a lower substrate having stripe projections asshown in FIG. 53 except that the rubbing directions r1 and r2 were setto angles of 0 degree and 180 degrees (anti-parallel), respectively. Thecell also provided a display with uniform and provided a display withuniform and rich gradation characteristic similarly as in Example 13.

Example 15

FIG. 56 illustrates a cell structure of a liquid crystal displayaccording to this Example.

An upper substrate included a glass substrate 1911 provided withpatterned ITO (transparent) electrodes 1912 and coated with a polyimidealignment film 1913. A lower substrate included a glass substrate 1914provided with patterned ITO (transparent) electrodes 1915, ITO stripeprojections 1916 formed thereon and a polyimide alignment film 1917. TheITO electrodes 1912 and 1915 were formed in a width of 200 μm and aspacing of 20 μm on both substrates. The stripe projections 1916 on thelower substrate were disposed in a width of 3 μm and at a spacingranging from 1 μm (at the densest part) to 15 μm (at the sparsest part).

A liquid crystal cell was prepared by using the two substrates otherwisein the same manner as in Example 11 (inclusive of preparation method,liquid crystal material and cell gap) and the rubbing direction r1 andr2 were also substantially parallel as in Example 11.

In this Example, the transparent electrodes and the stripe projectionswere were continuous over the adjacent pixels inclusive of the spacingtherebetween. This arrangement provided a better uniformity by rubbing.

The thus prepared cell was filled with the liquid crystal material ofExample 11 by heating the liquid crystal material to isotropictemperature and injecting the material in the direction of the stripesso as to suppress the injection irregularity to the minimum.

The thus obtained cell showed a good alignment state. The cell wasprovided with cross nicol polarizers set to a position so that one ofthe bistable states remoter to the stripe direction provided a resetposition (black state) and then driven by applying different pulsevoltages in the same manner as in Example 11, whereby a good gradationaldisplay characteristic was obtained. Further, the position of the crossnicol polarizers was shifted so that one of the bistable states ofliquid crystal molecules closer to the stripe direction provided a resetposition (black state). The cell was then driven by applying differentvoltages, whereby a similarly good gradational display characteristicwas obtained.

As described above, according to the present invention, it is possibleto effect a halftone or gradational display with a desired appliedsignal-transmittance characteristic (γ-characteristic) while retaining agood reproducibility of halftone display. Further, a good halftonedisplay can be effected at a high speed, an increased number ofgradation levels and a high definition without requiring substantialcomplication of the device structure.

Example 16

In this Example, a liquid crystal optical device according to thepresent invention as shown in FIG. 18 was prepared.

First, 1.1 mm-thick glass plates 11 and 12 were coated with about 700Å-thick ITO films 13 and 14 by sputtering to form transparent electrodesubstrates 10a and 10b.

Then, ultrafine particle mixture of tin oxide-antimony (SnO₂ --Sb)having an average particle size of 50 Å was dispersed in a siloxane-typepolymer dispersion to form a dispersion liquid, which was then appliedby a spinner rotating at about 1000-3000 rpm onto the transparentelectrode substrates 10a and 10b, followed by heating at 150° C. for 60min. to form about 1000 Å-thick polysiloxane films 3a and 3b containing70 wt. % the above ultra fine particles. Incidentally, the filmthickness may be controlled within a range of 100 Å to several thousandÅ (e.g., 3000 Å) by controlling the polymer dispersion density and thespinner coating conditions. Further, the film conductivity and otherelectrical properties may be varied widely by changing the mixing ratioof the ultrafine particles and the dispersion ratio thereof within thepolymer dispersion liquid.

Then, the fine particle-dispersion films 3a and 3b were coated with adiluted solution of polyimide film-forming liquid ("LQ-1802") having aresin concentration of about 0.8 wt. % by a spinner rotating at 2000 rpmfor 20 sec, followed by drying at 80° C. and baking at 270° C. for 1hour, to form about 30 Å-thick alignment control films 4a and 4b.

The above film thickness was first confirmed with respect to a polyimidefilm formed on a chromium plate under the same conditions as above bymeasurement using an automatic ellipsometer (available from GardnerCo.), and then the above-coated film on the device was directly observedthrough an electron microscope to confirm the presence of pinholeseverywhere within the polyimide film and an average film thickness ofabout 30 Å at the film-retaining portion.

Further, the surfaces of the polyimide films 4a and 4b were rubbed and,then onto one of the polyimide films, a dispersion liquid containing 1.4μm-dia. silica beads was applied and dried. On the other substrate, anepoxy adhesive was applied at prescribed parts, followed by light degreeof drying. Then, the pair of substrates were disposed and applied toeach other so as to form a parallel-rubbing cell (FIG. 20(a)), which wasthen filled with an ferroelectric liquid crystal material having aspontaneous polarization of about 7 nC/cm² by injection under vacuum.

The thus prepared liquid crystal cell showed a uniform alignment state.When the cell was subjected to a matrix gradational drive, a goodgradational characteristic was attained with little fluctuation betweenpixels (A and B) as shown in FIG. 22.

Example 17

A pair of substrates were treated up to formation of the alignmentcontrol films in the same manner as in Example 16 and the substrateswere applied to each other so as to form an anti-parallel cell, whichwas then filled with the same ferroelectric liquid crystal material asin Example 16. Immediately after the liquid crystal injection, theliquid crystal assumed a splay state over the entire cell area in somecases. However, if the cell was gradually cooled from the SmA phase toSm*C phase of the ferroelectric liquid crystal while applying arectangular AC electric field of 30 volts and 10 Hz, the ferroelectricliquid crystal assumed a uniform alignment state having a highhomogeneity and provided an increased apparent switching tilt anglebetween the memory states of the ferroelectric liquid crystal molecules.

When the cell was subjected to a gradational drive similarly as inExample 16, the cell showed a good gradational display characteristic asrepresented by a solid line in FIG. 57 which was further better thanthat of Example 16 as represented by a dashed line in FIG. 57.

Comparative Example 1

A liquid crystal cell was prepared in the same manner as in Example 17except that the fine particle-dispersion films 3a and 3b were notformed.

The cell, even after the AC application treatment in the same manner asin Example 17, could only provide alignment characteristic and gradationcharacteristic which were clearly inferior to those of Example 17.

Example 18

In this Example, a liquid crystal optical device as shown in FIG. 24 wasprepared.

Similarly as in Example 16, 1.1 mm-thick glass plates were coated withabout 700 Å-thick ITO films 13 and 14 by sputtering to form transparentelectrode substrates 10a and 10b, and then coated with about 1000Å-thick films of SiO₂, which were then patterned to leave 5 μm-squareprojections 7 at a pitch of 20 μm within a pixel area of 200 μm-square.

Then, on the substrates, the fine particle-dispersion films 3a and 3b,and the polyimide alignment control films 4a and 4b were formed in thesame manner as in Example 16, whereby pinholes of the polyimide films 4aand 4b were observed everywhere on the fine particle-dispersion films 3aand 3b.

After rubbing the polyimide films 4a and 4b, a cell was prepared in thesame manner as in Example 16 and filled with the same ferroelectricliquid crystal material under vacuum.

The thus prepared device of this Example showed a uniform alignmentstate and showed a further uniform gradational characteristic comparedwith Example 16.

According to Examples 16-18 as described above, it is possible toprovide a liquid crystal optical device suitable for use as a staticpicture or motion picture display panel having a large area inclusive ofa large number of pixels and suitable for gradational drive uniform andgood characteristics.

FIG. 58 shows an example of an image display apparatus inclusive of anexample of the liquid crystal display device (LC panel) according to thepresent invention. The apparatus includes a LC panel with 500×500 matrixpixels, a clock pulse generator 1802, a synchronizing circuit 1803, ascanning signal waveform generator 1806 inclusive of a shift register1804 and an analog switch 1805, and a data signal generator 1808 forreceiving image data from, e.g., a frame memory 1807, and converting theimage data into drive signals. These circuits may be disposed on eitherone or both of upper and lower sides, and/or on either one or both ofleft and right sides of a matrix substrate loaded with or constitutingthe LC panel. Data signal waveforms carrying halftone signals may beapplied as voltage-modulated signals as an ordinary method of providinggradational data. However, it is also possible to usepulse-width-modulated signals or phase modulated signals. This isparticularly effective when domains coupled by elastic propagation inchiral smectic C layer direction are utilized for domain growth, so asto control the propagation time.

What is claimed is:
 1. A liquid crystal display device, comprising:apair of oppositely disposed substrates each having a plurality ofopposing electrodes, and a ferroelectric liquid crystal disposed betweenthe substrates so as to form a plurality of pixels each composed by acombination of a pair of the opposing electrodes and the ferroelectricliquid crystal disposed therebetween; wherein each pixel is providedwith regions of different polarity inversion threshold voltages, and atleast one of said pair of opposing electrodes is provided with aplurality of regions having island-shaped unevennesses at differentdensities including a region with a higher density of unevennessescorresponding to a region of a lower polarity inversion thresholdvoltage and a region with a lower density of unevennesses correspondingto a region of a higher polarity inversion threshold voltage.
 2. Adevice according to claim 1, wherein the unevennesses comprise a part ofthe opposing electrode.
 3. A device according to claim 1, wherein theunevennesses comprise a conductor formed on the opposing electrode.
 4. Adevice according to claim 1, wherein the unevennesses comprise aninsulating member formed on the opposing electrode.
 5. A liquid crystaldisplay device, comprising a pair of opposing electrodes and aferroelectric liquid crystal disposed between the opposingelectrodes;wherein at least one of the opposing electrodes is providedwith a means for causing a lowering in polarity inversion thresholdvoltage of the ferroelectric liquid crystal in the form of a line, andat least one of the opposing electrodes is provided with island-shapedunevenesses disposed in a line as a means for causing a change in shapeof a polarity-inverted region in direction of the line of loweredpolarity inversion threshold voltage.
 6. A liquid crystal displaydevice, comprising: a pair of oppositely disposed substrates each havinga plurality of opposing electrodes, and a chiral smectic liquid crystaldisposed between the substrates so as to form a plurality of pixels eachcomposed by a combination of a pair of the opposing electrodes and thechiral smectic liquid crystal disposed therebetween;wherein each pixelis provided with regions of different polarity inversion thresholdvoltages, and at least one of said pair of opposing electrodes isprovided with a plurality of regions having unevennesses at differentdensities including a region with a higher density of unevennessescorresponding to a region of a lower polarity inversion thresholdvoltage and a region with a lower density of unevennesses correspondingto a region of a higher polarity inversion threshold voltage, and theunevennesses comprise projections having substantially equal heights. 7.A device according to claim 6, wherein the unevennesses areisland-shaped.
 8. A device according to claim 6, wherein theunevennesses are stripe-shaped.
 9. A device according to claim 7 or 8,wherein the unevennesses comprise a part of the opposing electrode. 10.A device according to claim 7 or 8, wherein the unevennesses comprise aconductor formed on the opposing electrode.
 11. A device according toclaim 7 or 8, wherein the unevennesses comprise an insulating memberformed on the opposing electrode.
 12. A liquid crystal display device,comprising a pair of opposing electrodes and a chiral smectic liquidcrystal disposed between the opposing electrodes, wherein at least oneof the opposing electrodes is provided with a means for causing alowering in polarity inversion threshold voltage of the chiral smecticliquid crystal in the form of a line, and at least one of the opposingelectrodes is provided with a means for causing a change in shape of apolarity-inverted region in direction of the line of lowered polarityinversion threshold voltage, wherein said means for causing a loweringin polarity inversion threshold voltage comprises a plurality ofprojections having substantially equal heights.
 13. A device accordingto claim 12, wherein said means for causing a lowering in polarityinversion threshold voltage comprises stripe-shaped unevennessesdisposed in parallel.
 14. A device according to claim 12, wherein saidmeans for causing a change in shape of a polarity-inverted regioncomprises island-shaped or stripe-shaped unevennesses disposed inalignment with the line of polarity inversion threshold voltage.
 15. Adevice according to claim 12, wherein said means for causing a change inshape of a polarity-inverted region comprises a means for causing anelectric field applied to the chiral smectic liquid crystal to graduallyincrease or decrease in a direction along the line of lowered polarityinversion threshold voltage.
 16. A device according to claim 15, whereinsaid means for causing an electric field applied to the chiral smecticliquid crystal to gradually increase or decrease in a direction alongthe line of lowered polarity inversion threshold voltage comprisesisland-shaped or stripe-shaped unevennesses arranged at a varyingdensity which increases or decreases in a direction of the line oflowered polarity inversion threshold voltage.
 17. A device according toclaim 15, wherein said means for causing an electric field applied tothe chiral smectic liquid crystal to gradually increase or decrease in adirection along the line of lowered polarity inversion threshold voltagecomprises a gradient formed in a direction along the line of loweredpolarity inversion threshold voltage.
 18. A device according to claim15, wherein said means for causing an electric field applied to thechiral smectic liquid crystal to gradually increase or decrease in adirection along the line of lowered polarity inversion threshold voltagecomprises a means for applying a bias voltage across one of the opposingelectrode in a direction along the line of lowered polarity inversionthreshold voltage.